Computer Based
Terrain Visualisation Techniques

"From Points to Wood"

Written by Paul Bourke
August 1988


Introduction

This report describes some of the computer based techniques which can be used to aid in the visualisation of terrain. In the following discussion it is assumed that we have acquired data in a suitable form which allows us to define the surface on a computer. This usually starts as a series of points lying on the surface and stored as x,y,z triples. There are many ways of getting this data, the example here was digitised from contour maps using a large scale digitising tablet connected to a computer. The next step is normally to turn the digitised data into a series of polygonally bounded planes, called facets, so that they can be read, viewed and manipulated by 3D modelling and rendering software. The point data is this example is transformed into a regular polygonal mesh by an algorithm called Delaunay triangulation See an earlier discussion by myself for details of this algorithm and computer program which transforms randomly distributed spot heights into triangular or regular meshes and exports this surface description in a format suitable for most 3D modelling and/or rendering programs.

Contouring

Contouring has been the traditional way of presenting 3D terrain data on 2D sheets of paper. It has advantages of having units (contour values) so precise height calculations can be made. Contour lines are usually drawn with line segments so they can easily be transferred to large scale hardcopy devices such as plotters. The main problem with this visualisation technique is that it does not give a good 3D impression of the terrain, the best most people can determine is that some localised parts of the surface are higher or lower than others.

Mesh representations

This is the most straightforward way of rendering the 3D data, it is a direct perspective viewing transformation of the computer database. (Almost any 3D modelling package can display, view, and print this representation, MicroStation was used here due to its advantages when handling very large geometric databases).


Figure 1
 

Figure 1 shows the example landscape represented in this way. The reasons why this makes the landscape look three dimensional are: the automatic shading of the steep parts of the terrain because the mesh lines appear closer together, the flat areas are obvious due to the regularity of the mesh, the perspective transformation gives a simple depth cueing due to the mesh lines becoming denser. These effects generally require that the mesh density approaches that of the viewing device, or at least that the projection of the mesh onto the viewing plane is close to the resolution of the device.

This rendering can also be in colour. The colour may be related to height but it could also be some other attribute such as ground cover, population, etc.

The example in Figure 1 is a 128x128 cell mesh and it consists of about 33,000 line segments. On many high end computer platforms this can be drawn in real time so that the user can "fly" about the landscape. This number of polygons can however cripple many desktop machines. For this reason good terrain modelling software allows the user to view the surface at a range of resolutions.

Vertical contouring

Figure 2
 

While horizontal contouring has long been a popular method, vertical contours can also be effective. The reasons for the success of this rendition method are the same as for mesh representations, it generates dark (dense) areas which would naturally be shadowed in direct sun light.

Figure 2 is an example of such a rendering of our demonstration landscape. This rendering was created with a 3D modelling package called Vision3D written by myself. There is a considerable reduction in the data needed to draw this form of rendering, unfortunately the effect is only satisfactory when the contour lines are about 45 degrees to the view direction and so animations are not usually possible.

Shaded renderings

Higher levels of realism can be achieved by simulating more closely how the terrain would appear in reality. There are a wide range of techniques for accomplishing this, each technique generally involves a trade off between realism and computation time.


Figure 3
 

Figure 3 shows a relatively sophisticated rendering This rendering was created using a raytracing/radiosity package called Radiance. It was chosen mainly because it was available on a very fast hardware platform. One undesirable characteristic of this technique can be seen in the patches visible on the surface. These are the result of the limits in the data available in the description of the surface. The courser the mesh the more obvious the patches are. The finer the mesh the more data the rendering process must handle and the longer it will take.

There are techniques (Phong shaded polygons) which can reduce and even remove the patchiness of the surface by interpolating the surface normals between the facets. These techniques however require more computation time and have the undesirable characteristic that the shadows are still based on the gridded geometry and do not appear smooth. The example in Figure 3 uses the same data density as the mesh shown in Figure 1.

Physical models

Given the digital model it is possible to determine the path required to manufacture the surface using a computer controlled milling machine. In its simplest form this is a drill which can be controlled backward and forward and up and down over a piece of wood say, so as to cut away unwanted portions.


Figure 4

Figure 4 shows the result of milling our example landscape from wood. I used a OH-FANUC, model 2R-NC milling machine operated by the School of Engineering, Auckland University. The machine is controlled by the MasterCAM software For this example the wood is about 300mm square and the drill bit used in about 5mm radius. Note that it is not necessary to use a particularly fine bit because the wood is cut avay at the edge of the bit not from the bottom. The bit size then only determines the narrowest valleys that are possible. This physical scale model of the landscape has big advantages for visualisation purposes. The model is to scale, although in this case there is a 2 times exaggeration in height. The viewer can instantaneously view the terrain from various positions and angles by simply turning the model about. Tactile exploration of the model is of course possible and can be informative as well as satisfying.

An extension of this technique would be to automatically draw features such as roads, boundaries, contours, etc onto the landscape. This could be done with a robot arm holding a pen or with a laser which would burn the line features onto the surface of the wood.